Implantable cardiac devices typically sense cardiac electrical signals in an implantee and classify the implantee's cardiac rhythm as normal/benign or malignant. Illustrative malignant tachyarrhythmias include ventricular fibrillation and polymorphic ventricular tachyarrhythmia. Other tachycardia or bradycardia conditions may be of interest as well. The accuracy with which an implantable medical device analyzes sensed signals determines how well it makes therapy determinations and other decisions. Incorrect rate calculation can lead to inappropriate classification of cardiac activity. For example, calculation of an erroneously high cardiac rate can cause a system to identify a cardiac arrhythmia that may not actually be occurring. Inappropriate classification can, in turn, lead to incorrect therapy decisions.